Clot lysis time determinations are important clinical tests. Clot lysis, or thrombolysis, occurs through fibrinolysis. In the body, fibrinolysis is carefully coordinated with coagulation so that prompt control of bleeding can occur through coagulation, and yet eventual resolution and healing can occur through fibrinolysis.
Blood clotting is a very complex series of events triggered by damage to a blood vessel. Clot formation is essential for preventing continuous bleeding. A clot is composed of platelet and fibrin, (predominately fibrin) as the clot matures. This is known as insoluble fibrin. However, sometimes clots form for other reasons and cause several problems including heart attacks, strokes, and embolisms. The clotting system is counter-acted by the fibrinolytic system naturally present in the body. This system breaks up the clot which is mostly composed of fibrin, thus the term fibrin-o-lysis. Lysis means break apart or degrade.
Fibrin formation (coagulation) and dissolution (fibrinolysis) are thought to occur simultaneously during wound clotting and healing. Studies of pathologic thrombosis support this concept of clot remodeling and indicate that the balance of the two opposing processes varies in different areas of a single large clot. Thus, there is a complex interaction between activation and inhibition of fibrinolysis.
Practitioners of medicine must be careful when altering the interaction of coagulation and fibrinolysis because pharmacological intervention can produce symptoms of bleeding and thrombosis, as well as correct symptoms of bleeding and thrombosis. For instance, disruption of the body's control of fibrinolysis can, by shifting the balance of clot formation and dissolution, lead to bleeding if there is inappropriate fibrinolytic inhibition. The bleeding caused by excess fibrinolytic stimulus can be corrected by administration of a fibrinolytic inhibitor. However, if the patient is predisposed to venous thrombosis or disseminated intravascular coagulation (DIC), clinical thrombosis could occur when the fibrinolytic inhibitor is given.
Bleeding may result either from defective inhibition, or from excessive activation of fibrinolysis. Conversely, either defective activation, or excessive inhibition of fibrinolysis can lead to thrombosis. The fact that therapy with fibrinolytic agents may dissolve a thrombus, and may also lead to bleeding, complicates clinical management. Also, the fact that antifibrinolytic therapy may stop bleeding due to excess fibrinolysis, or may lead to thrombotic complications, complicates clinical management. For instance, stimulation of fibrinolysis through therapeutic administration of plasminogen activator for treatment of thrombosis may result in uncomplicated thrombolysis, or may result in bleeding complications in susceptible patients.
Thus, the need to accurately determine the fibrinolytic capabilities of a patient's blood is critically important. Successful therapy, that is, restoration of effective hemostasis or achievement of thrombolysis without complications depends critically on selection of the appropriate treatment for the patient. Clearly, the balance between coagulation and fibrinolysis influences the occurrence and clinical course of thrombotic disease.
The fibrinolytic system is much less understood then the clotting system. The fibrinolytic system is activated through the release of tissue plasminogen activator from blood vessel endothelial cells. Tissue plasminogen activator (t-PA) activates plasminogen on the surface of clots. Fibrin degradation products (FDPs) are released from the lysed clot when it is digested. FDPs are small fragments which result from the digestion of fibrin. The FDPs, called D, X, Y, and E fragments, can be measured. These fragments, along with fibrin degradation products, fibrinogen, and plasmin, are all part of the fibrinolysis cascade and are the only elements measured. They are the end result of fibrin degradation. Low levels of FDPs, seen in the serum of patients after surgery or trauma, appear to have little effect on the coagulation systems. However, fragment degradation products seen in DIC or with fibrinolytic therapy may have an inhibitory effect on the bleeding time and coagulation screening tests. These products may also contribute to the defect in fibrin formation in these patients.
Fibrinolytic therapy with tissue plasminogen activator, urokinase, or streptokinase can cause lysis of fibrin as well as lysis of fibrinogen. Fibrin degradation products are distinguished from fibrinogen degradation products by the presence of Factor XIII cross-linkage of fibrin D domains. Commercial kits for immunologic detection of "D--D" dimers are available to detect a "segment or portion" of fibrinolysis. Currently, there are techniques available to measure static parameters of fibrinolysis, such as the end results of the fibrinolytic pathway (e.g., D-dimers), but there is nothing to measure the dynamics of fibrinolysis.
Rheology is a general term for the study of blood flow or any deformation of flow. Rheology is pertinent for coagulation processes. There are three important characteristics of blood clotting and lysis of the clot. These characteristics are: (1) the vascular integrity (e.g., structure--internal and external elastic membrane of a vessel, and function--the endothelial cells, smooth muscle cells, and fibroblasts); (2) the blood within the vessel (e.g., the cells, coagulation proteins, inhibitors, etc.); and (3) the flow of blood through the vessels and across the endothelial cells.
Among the current tests for determining the fibrinolytic capability of a fluid are tests which involve measurement of the levels present of various chemicals and proteins involved in the fibrinolytic pathway such as fibrinogen and factor XIII, or measurement of the chemicals and proteins involved in the plasmin/plasminogen cascade that induces lysis of fibrin. The chemicals and proteins involved in the fibrinolysis cascade include plasminogen, tissue plasminogen activator, serine protease plasmin, tissue plasminogen activator inhibitor, and alpha 2-antiplasmin. The total concentration of these chemicals and proteins can be measured immunologically or by chromogenic assays. The degree of fibrinolysis can be assessed indirectly by the measurement of fibrin degradation products.
Currently, screening tests for fibrinolysis are extremely limited. The thrombin clotting time will detect an abnormality in fibrinogen, but otherwise chromogenic and immunological assays of specific proteins must be performed. A diagnosis of an increased fibrinolytic state requires further assessment, often beginning with the global clot lysis time. Clot Lysis Time (CLT) measures the action of plasminogen activators and plasmin in the blood and is usually performed on clots formed from the euglobulin fraction of the plasma or whole blood. In these tests, inhibitors of activators and plasmin are removed by precipitation of plasma or whole blood at low ionic strength and low pH. After removal of the supernatant fluid, the precipitate is resuspended in an appropriate buffer and clotted with thrombin, and the time required for clot lysis is recorded. Abnormally short lysis times (less then 2 hours) reflect acute episodes of excessive fibrinolysis that accompany a variety of acquired disorders or that result from the administration of plasminogen activators such as streptokinase. CLT is currently not a useful screening test due to the time necessary to obtain results.
A problem concerning the handling of blood, and blood-derived products, is health safety. Blood samples may carry sexually transmitted infectious agents such as HIV 1 and 2, Hepatitis B, syphilis, etc. which could infect the clinician if proper safeguards are not in place.